1. Field of the Invention
This invention relates to recording methods for disc-shaped recording media, reproducing apparatuses for disc-shaped recording media, and disc-shaped recording media. In particular, this invention relates to a method for recording to a disc-shaped recording medium in which a digital audio signal is recorded, an apparatus for reproducing from a disc-shaped recording medium in which a digital audio signal is recorded, and a disc-shaped recording medium in which a digital audio signal is recorded.
2. Description of the Related Art
Compact discs are in wide use as a type of recording medium to which an audio signal is converted and recorded in the form of a digital signal or from which medium such an audio signal can be reproduced. According to the standard for compact discs, an analog audio signal is sampled corresponding to each right or left channel at a sampling frequency of fs=44.1 kHz, and each sample is converted and recorded as a 16-bit digital signal.
In the compact disc, an analog audio signal which is to be recorded corresponding to each channel is processed such that its frequency band of not less than 22 kHz is filtered due to the above-mentioned limited sampling rate in the standard. Therefore, the harmonics originally included in a sound are limited, which thus causes a deterioration in tone quality such as a lack of natural sense.
Another standard in which a sampling frequency of 96 kHz corresponding to each channel where each sample is recorded as a 24-bit signal has been proposed. Although some improvement has been observed, deterioration in tone quality cannot be avoided even when this standard is employed, similar to the former standard due to limited harmonics.
In addition, recently for analog-to-digital or digital-to-analog converters, a converter employing the .SIGMA..increment.-modulation method in which quantized noise is concentrated within a high frequency by providing a quantizer in a feedback loop, has been widely noticed. This converter is used for recording to or reproducing from compact discs.
FIG. 1 is a system block diagram showing the flow of an audio signal recorded or reproduced with respect to a compact disc using the .SIGMA..increment.-modulation method, based on the standard for compact discs.
A .SIGMA..increment.-converter 11 converts an analog signal "Au" into a digital audio signal "Da" having 1 bit per sample (this digital audio signal having 1 bit per sample is hereinafter referred to as the "1-bit audio signal") by using a sampling frequency of 64.multidot.fs (=2.8224 MHz). The 1-bit audio signal is supplied to a digital filter 12. This digital filter 12 performs thinning-out (down-sampling) with respect to the supplied signal for conversion into a digital audio data "Db" having 16 bits per sample by using the sampling frequency denoted by fs, which data satisfies the standard for compact discs.
The digital audio data Db is supplied to an encoding circuit 13, in which processes such as scrambling and error-correcting-encoding and eight-to-fourteen modulation are performed with respect to the supplied data, so that the processed and modulated data is recorded as serial data by a recording optical head (not shown) to a disc-shaped recording medium 14.
The serial audio data recorded to this disc-shaped recording medium 14 is read by a reproducing optical head (not shown) to be supplied to a decoding circuit 15. This decoding circuit 15 converts the serial data into parallel data, and performs processes such as de-scrambling and error-correcting-decoding to form reproduced digital audio data "Dbp" having 16 bits per sample by using the sampling frequency fs.
This reproduced digital audio data Db is supplied to a digital filter 16, in which interpolation (over-sampling) is performed with respect to the supplied data in response to the thinning-out on the recording side, and the interpolated output data is supplied to a .SIGMA..increment.-modulator 17. The .SIGMA..increment.-modulator 17 converts the supplied data into a 1-bit audio signal "Dap" by using a sampling frequency of 64.times.fs.
The 1-bit audio signal Dap from the .SIGMA..increment.-modulator 17 is supplied to an analog low-pass filter 18 to be restored to an analog audio signal "Aup".
When the .SIGMA..increment.-modulation is used in analog-to-digital and digital-to-analog conversion as described above, a wide dynamic range is obtained with a small number of bits by increasing the sampling frequency.
However, according to the case shown in FIG. 1, in the recording mode the digital filter 12 performs thinning-out with respect to the digital audio data having 16 bits per sample at the sampling frequency fs based on the standard for compact discs, while in the reproducing mode the digital filter 16 performs interpolation. Accordingly, a computing error occurs in the process in which the data passes through the digital filters 12 and 16, and this error causes a deterioration in tone quality.
On the other hand, with the development for practical use of media having a large capacity and a high transfer rate, recording methods and apparatuses for a digital audio signal sampled at a greater number of bits and at a higher frequency have been proposed, compared with the present standard for compact discs at a sampling frequency of fs=44.1 kHz and 16 bits per sample. Such circumstances cause increasing dissatisfaction with the tone quality of the compact disc, and as a result, enthusiasm for realizing the next-generation of compact discs has increased.
As the next-generation of compact discs will not be limited to audio-use, Toshiba Corporation and Matsushita Electric Industrial Co., Ltd. in Japan have proposed a type of disc for realizing a recording capacity of 5 GB on one surface (referred to as the "SD (super density disc) standard" hereinafter), where the disc has a thickness of 1.2 mm formed by cementing two thin discs being 12 cm in diameter and 0.6 mm thick, similar to the present compact disc. Also, Phillips in the Netherlands and Sony Corporation in Japan have proposed another type of disc for realizing a recording capacity of 3.7 GB on one surface (referred to as the "MMCD (multi-media compact disc)" hereinafter), which has a single-disc structure of the same size as the present compact disc. In addition, a further type of disc realizing an intermediate recording capacity of 4.7 GE on one surface, between the SD standard and the MMCD standard, has been proposed.
These types of discs have been proposed as digital video discs (referred to as "DVDs", hereinafter) or data memories for computers.